A dual near-infrared pH fluorescent probe and its application in imaging of HepG2 cells

Covalent Organic Frameworks Doped with Different Ratios of OMe/OH as Fluorescent and Colorimetric Sensors

Improving the luminescence properties of covalent organic frameworks (COFs) has always been an important issue. Here, a series of COFs (([OMe]_x -TzDa (TzDa is composed only by monomerics Tz and Da, OMe represents the incorporation of monomeric Dm)) with different ratios of OMe and OH were designed and synthesized. The photochemical behavior of [OMe]_x -TzDa changed significantly due to the synergistic effect of aggregation induced emission (AIE), intramolecular charge transfer (ICT), and excited-state intramolecular proton transfer (ESIPT) effects. [OMe]₂ -TzDa, which contained a ratio of 2/1 of OMe/OH, showed the strongest fluorescence emission in water and the best linear relationship for the detection of pH. Furthermore, [OMe]₂ -TzDa was used to monitor HCl and NH₃ gases and showed a color change, visible to the naked eye. Therefore, a "confidential pigment" was successfully made. Moreover, [OMe]₂ -TzDa was also applied to detect N₂ H₄ . The work indicates the [OMe]₂ -TzDa can serve as the first fluorescence sensor to detect pH, HCl and NH₃ gases, which also shows a good response to N₂ H₄ .

A new near-infrared fluorescent probe for sensing extreme acidity and bioimaging in lysosome

Since the intracellular pH plays an important role in the physiological and pathological processes, however, the probes that can be used for monitoring pH fluctuation under extreme acidic conditions are currently rare, so it is necessary to construct fluorescent probes for sensing pH less than 4. In this work, we developed a new near-infrared (NIR) fluorescent probe Cy-SNN for sensing pH fluctuation under extremely acidic conditions. For the preparation of this probe, benzothiozolium moiety was chosen as lysosomal targeting unit and NIR fluorophore, and barbituric acid moiety was fused in the polymethine chain of probe to introduce protonation center. Surprisingly, on the basis of the balance of quaternary ammonium salts and free amines, the pKa value of Cy-SNN was calculated as low as 2.96, implying that Cy-SNN can be used in acidic conditions with pH < 4. Moreover, Cy-SNN exhibited highly selective response to H+ over diverse analytes in real-time with dependable reversibility. Importantly, Cy-SNN can be used to specifically target lysosome, providing potential tools for monitoring the function of lysosome in autophagy process.

Spectral Characterization of Purpurin Dye and Its Application in pH Sensing, Cell Imaging and Apoptosis Detection

Purpurin (1,2,4-trihydroxy-9,10-anthraquinone) is a natural red dye obtained from the red madder plant that is widely used in food and dyeing industries. The present study investigated the characteristics of purpurin and its application as a pH-sensitive probe to detect the pH of solutions and intracellular pH of mammalian and bacterial cells. Purpurin exhibited high pH-sensitive behavior, low analytes interference, high stability with pKa of 4.6 and visible colorimetric change. 1H NMR and FTIR studies indicated protonation of phenolic hydroxyl group under acidic condition with hypsochromic shift in the absorption and fluorescence spectra relative to that of basic condition. Cell culture studies using HeLa cells revealed that purpurin is well tolerated by the cells and the fluorescent imaging result indicated excellent cell permeability with possible use of the dye to detect the pH fluctuations in living cells under various physiological conditions such as apoptosis. Microbiological studies indicated that the dye could be used for visualization of bacteria under acidic condition.

Emergent Biosensing Technologies Based on Fluorescence Spectroscopy and Surface Plasmon Resonance

The purpose of this work is to provide an exhaustive overview of the emerging biosensor technologies for the detection of analytes of interest for food, environment, security, and health. Over the years, biosensors have acquired increasing importance in a wide range of applications due to synergistic studies of various scientific disciplines, determining their great commercial potential and revealing how nanotechnology and biotechnology can be strictly connected. In the present scenario, biosensors have increased their detection limit and sensitivity unthinkable until a few years ago. The most widely used biosensors are optical-based devices such as surface plasmon resonance (SPR)-based biosensors and fluorescence-based biosensors. Here, we will review them by highlighting how the progress in their design and development could impact our daily life.

A novel near-infrared fluorescent probe with large stokes shifts for sensing extreme acidity and its application in bioimaging

In this work, we reported a novel near-infrared (NIR) fluorescent probe RQNN with large Stokes shift (98 nm) for monitoring pH under extremely acidic conditions. For the preparation of this probe, a 1,4-diethylpiperazine moiety was introduced in rhodamine scaffold to tune the electron-donating character, and an o-phenylenediamine was introduced in spironolactone to provide larger steric hindrance. The deprotonated-protonated equilibrium between RQNN, RQNN-H⁺ and RQNN-H⁺⁺ were evaluated in different pH by absorption and emission spectra. As expected, RQNN exhibited lower pk_a values (pk_a1 = 4.83, pk_a2 = 2.99), indicating that the probe can be used in extremely acidic pH. Moreover, RQNN possessed highly selective response to H⁺ over essential metal ions and biologically related redox molecules, high photo-stability, rapid response time, and excellent reversibility. Importantly, the probe had excellent cell membrane permeability and was further applied successfully to monitor pH fluctuations in live cells.

Research progress of near-infrared fluorescence probes based on indole heptamethine cyanine dyes in vivo and in vitro

Near-infrared (NIR) fluorescence imaging is a noninvasive technique that provides numerous advantages for the real-time in vivo monitoring of biological information in living subjects without the use of ionizing radiation. Near-infrared fluorescent (NIRF) dyes are widely used as fluorescent imaging probes. These fluorescent dyes remarkably decrease the interference caused by the self-absorption of substances and autofluorescence, increase detection selectivity and sensitivity, and reduce damage to the human body. Thus, they are beneficial for bioassays. Indole heptamethine cyanine dyes are widely investigated in the field of near-infrared fluorescence imaging. They are mainly composed of indole heterocyclics, heptamethine chains, and N-substituent side chains. With indole heptamethine cyanine dyes as the parent, introducing reactive groups to the parent compounds or changing their structures can make fluorescent probes have different functions like labeling protein and tumor, detecting intracellular metal cations, which has become the hotspot in the field of fluorescence imaging of biological research. Therefore, this study reviewed the applications of indole heptamethine cyanine fluorescent probes to metal cation detection, pH, molecules, tumor imaging, and protein in vivo. The distribution, imaging results, and metabolism of the probes in vivo and in vitro were described. The biological application trends and existing problems of fluorescent probes were discussed.

Design Strategies of Photoacoustic Molecular Probes

Photoacoustic (PA) probes have been developed very quickly and applied in broad areas in recent years. Most of them are constructed based on organic dyes with intrinsic near-infrared (NIR) absorption properties. To increase PA contrast and improve imaging resolution and the sensitivity of detection, various methods for the design of PA probes have been developed. This minireview mainly focuses on the development and design strategies of activatable small-molecule PA probes in four aspects: reaction-cleavage, metal ion chelation, photoswitch, and protonation-deprotonation. It highlights some key points of designing PA probes corresponding to their properties and applications. The challenges and perspectives for small-molecule PA probes are also discussed.

Bis-cyclometallated Ir(III) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands; influence of substituents and cyclometallating ligands on response to changes in pH

Bis-cyclometallated Ir(iii) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands have been synthesised. Their absorption is almost unchanged with changes in pH however the emission intensities vary by a factor of up to three and the complexes have emission pKas in the range 8.0 to 10.0. Substituents on the pyrazole have only a minor effect on the emission pKa. Surprisingly the complexes with phenylpyrazole cyclometallated ligands 3aL1-3 showed an intensity decrease with increasing pH (switch off) whilst the corresponding phenylpyridine ones 3cL1-3 showed an increase in emission intensity with increasing pH. Putting electron-withdrawing CF3 substituents on the cyclometallating phenyls reduced the pKa of the complexes to 6.8-7.8, thereby extending the useful pKa range; however, in general it tended to reduce the magnitude of the change in emission intensity. Surprisingly the CF3-substituted complexes also showed a complete reversal in the direction of the intensity change when compared to their respective unsubstituted congeners.

A sensitive pH fluorescent probe based on triethylenetetramine bearing double dansyl groups in aqueous solutions and its application in cells

pH fluorescent probes possess many advantages, including intracellular detection, rapid response time and nondestructive testing. In this paper, a highly selective and sensitive fluorescent pH probe based on triethylenetetramine bearing double dansyl groups (1) was synthesized. This probe offers fluorescent measurement of pH value in the range of 5.81-7.21 in aqueous solution, with an 8.64-fold enhancement of fluorescent emission intensity over the unmodified probe. Probe 1 shows a fluorescent color change from a pale yellow to bright green when the pH is increased from 5.81 to 7.21. In addition, probe 1 shows good potential as a fluorescent visualizing sensor for pH values in living GS cells of epinepheluscoioides. The mechanism of the fluorescent response of probe 1 to solution pHs was further clarified by NMR, fluorescent spectra, and UV-vis absorption spectra. The results indicate that the fluorescent emission will shift with an increase in solution pHs, due to increasing deprotonation of the nitrogen atom on the sulfonamides. Deprotonation of the sulfonamide group will inhibits the intramolecular charge transfer process between the imino group and the naphthalene ring, resulting in the recognition phenomenon of blue shift and enhancement of fluorescent emission intensity.

Novel near-infrared fluorescent probe for live cell imaging

Near infrared (NIR) fluorescent probes play a crucial role in biological system imaging. A novel NIR fluorescent probe, IR787, was designed in the present study. Compared with indocyanine green (ICG), IR787 showed lower background fluorescent interference and higher fluorescence enhancement. Fluorescence intensities were detected by a Cary Eclipse fluorescence spectrophotometer. The interference of intracellular ions (Cu²⁺, Ca²⁺, Mg²⁺ and Zn²⁺) on the measurement was negligible, which indicated a good photostability of IR787. MTT assay demonstrated that cell viability of human lung adenocarcinoma epithelial cell line A549 was not significantly affected by the use of the IR787 probe compared with the ICG probe. This result suggested that the IR787 probe was safe for in vitro cell imaging. In vitro NIR optical imaging experiments further revealed cellular uptake and strong intracellular NIR fluorescence of the IR787 probe in A549 cells. The excitation wavelength was 787 nm for IR787. Compared with the previously reported NIR fluorescent probe ICG, the IR787 NIR fluorescent probe had improved prospects for intracellular imaging. IR787 may play a pivotal role in the understanding cell biology, pharmacology and disease diagnosis.

Monitoring mitochondrial pH with a hemicyanine-based ratiometric fluorescent probe

Mitochondria as essential organelles play critical roles in cellular metabolism. Mitochondrial pH is a vital parameter that directly affects the unique function of mitochondria. Herein, we present a new ratiometric fluorescent probe M-pH for monitoring the pH within the mitochondria. M-pH consists of a stable and large π-electron conjugated merocyanine system. The lipophilic cationic benzyl group will facilitate the accumulation of M-pH in mitochondria. The phenol unit is the recognition moiety, achieving the ratiometric sensing of pH changes. The experimental results indicate that M-pH displays ratiometric fluorescence response to different pH values. Meanwhile, M-pH shows negligible response to common species, and has high stability and low cytotoxicity. In biological experiments, M-pH can solely accumulate in mitochondria and visualize the pH changes during mitophagy and cell apoptosis. We thus believe that M-pH has great potential as a practical tool for real-time monitoring of pH changes of mitochondria, contributing to revealing the pathogenesis of mitochondrial pH associated diseases.

Molecular engineering of a dual emission near-infrared ratiometric fluorophore for the detection of pH at the organism level

A near-infrared ratiometric fluorophore (NIR-HBT) was rationally designed and constructed by expanding both the excitation and emission wavelength of the classical ratiometric fluorophore 2-(benzothiazol-2-yl)phenol (HBT) into the near-infrared region. The NIR-HBT was easily synthesized by incorporating the HBT module into the hemicyanine skeleton and showed evident NIR ratiometric fluorophore characteristics. Further application of the new fluorophore for pH detection demonstrated that NIR-HBT possesses superior overall analytical performance and NIR-HBT was successfully applied for detection of acidosis caused by inflammation in living animal tissue, which indicated the potential application value of NIR-HBT in biological imaging and sensing.

Fluorescent probes and bioimaging: alkali metals, alkaline earth metals and pH

All living species and life forms have an absolute requirement for bio-functional metals and acid-base equilibrium chemistry owing to the critical roles they play in biological processes. Hence, a great need exists for efficient methods to detect and monitor biometals and acids. In the last few years, great attention has been paid to the development of organic molecule based fluorescent chemosensors. The availability of new synthetic fluorescent probes has made fluorescence microscopy an indispensable tool for tracing biologically important molecules and in the area of clinical diagnostics. This review highlights the recent advances that have been made in the design and bioimaging applications of fluorescent probes for alkali metals and alkaline earth metal cations, including lithium, sodium and potassium, magnesium and calcium, and for pH determination within biological systems.

A novel fluorescent sensor based on imidazo[1,2-a]pyridine for Zn2+

A novel chemosensorL1for the detection of Zn²⁺based on imidazo[1,2-a]pyridine in C₂H₅OH–H₂O (9 : 1, v/v) buffer solution was designed and synthesized.

Nanoparticle-based luminescent probes for intracellular sensing and imaging of pH

Fluorescence imaging microscopy is an essential tool in biomedical research. Meanwhile, various fluorescent probes are available for the staining of cells, cell membranes, and organelles. Though, to monitor intracellular processes and dysfunctions, probes that respond to ubiquitous chemical parameters determining the cellular function such as pH, pO2 , and Ca(2+) are required. This review is focused on the progress in the design, fabrication, and application of photoluminescent nanoprobes for sensing and imaging of pH in living cells. The advantages of using nanoprobes carrying fluorescent pH indicators compared to single molecule probes are discussed as well as their limitations due to the mostly lysosomal uptake by cells. Particular attention is paid to ratiometric dual wavelength nanosensors that enable intrinsic referenced measurements. Referencing and proper calibration procedures are basic prerequisites to carry out reliable quantitative pH determinations in complex samples such as living cells. A variety of examples will be presented that highlight the diverseness of nanocarrier materials (polymers, micelles, silica, quantum dots, carbon dots, gold, photon upconversion nanocrystals, or bacteriophages), fluorescent pH indicators for the weak acidic range, and referenced sensing mechanisms, that have been applied intracellularly up to now. WIREs Nanomed Nanobiotechnol 2016, 8:378-413. doi: 10.1002/wnan.1366 For further resources related to this article, please visit the WIREs website.

Cyanine polyene reactivity: scope and biomedical applications

Cyanines are indispensable fluorophores that form the chemical basis of many fluorescence-based applications. A feature that distinguishes cyanines from other common fluorophores is an exposed polyene linker that is both crucial to absorption and emission and subject to covalent reactions that dramatically alter these optical properties. Over the past decade, reactions involving the cyanine polyene have been used as foundational elements for a range of biomedical techniques. These include the optical sensing of biological analytes, super-resolution imaging, and near-IR light-initiated uncaging. This review surveys the chemical reactivity of the cyanine polyene and the biomedical methods enabled by these reactions. The overarching goal is to highlight the multifaceted nature of cyanine chemistry and biology, as well as to point out the key role of reactivity-based insights in this promising area.

Synthesis of mono-(p-dimethylamino)styryl-containing BOPHY dye for a turn-on pH sensor

Mono-substitutional BOPHY 3a with a (p-dimethylamino)styryl group in the α-position was confirmed to be synthesized by the Knoevenagel-type condensation. Dimethylamino-containing BOPHY 3a can be used as a pH probe.

Dansyl-8-aminoquinoline as a sensitive pH fluorescent probe with dual-responsive ranges in aqueous solutions

A sensitive pH fluorescent probe based on dansyl group, dansyl-8-aminoquinoline (DAQ), has been synthesized. The probe showed dual-responsive ranges to pH changes, one range from 2.00 to 7.95 and another one from 7.95 to 10.87 in aqueous solution, as it showed pKa values of 5.73 and 8.56 under acid and basic conditions, respectively. Furthermore, the pH response mechanism of the probe was explored successfully by using NMR spectra. The results indicated that the responses of DAQ to pH changes should attribute to the protonation of the nitrogen atom in the dimethylamino group and deprotonation of sulfonamide group.

Synthesis of a near-infrared fluorescent probe and its application in imaging of MCF-7 cells

IR-789, a novel near-infrared fluorescent probe, was designed, synthesized, and applied to living cells. The probe exhibited better response fluorescence characteristics than the only FDA-approved agent, indocyanine green. Cell experiments showed that the probe had high affinity and without apparent cytotoxicity. Fluorescent image experiments in living MCF-7 cells (human breast adenocarcinoma cell line) further demonstrated the potential applications of the probe in biological systems. The probe effectively prevented the influence of autofluorescence and native cellular species in biological systems. It also exhibited high sensitivity, good photostability, and excellent cell membrane permeability.

Novel fluorescent sensors based on benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid for Cu2+

(1) Two new fluorescent sensors were developed based on benzimidazo[2,1-a]benz[de]isoquinoline-7-one. (2) BothC1andC2showed fluorescence turn-on response toward Cu²⁺in buffer solutions. (3) The detection limit of C1 towards Cu²⁺reached 5.7 × 10⁻⁸M. (4) The Cu²⁺sensing ofC1andC2were both based on the PET process. (5) Addition of OH⁻could lead to significantly color changes ofC1solution.

Colorimetric and ratiometric pH responses by the protonation of phenolate within hemicyanine

The indolium–phenol based tetramethylene hemicyanine has colorimetric and ratiometric optical responses under acidic and basic conditions.